Calcium phosphates are the principal constituent of hard tissues (bone, cartilage, tooth enamel and dentine). Naturally-occurring bone mineral is made of nanometer-sized, poorly-crystalline calcium phosphate with an apatitic structure. The poorly crystalline apatitic calcium phosphate of bone is distinguished from the more crystalline hydroxyapatites and non-stoichiometric hydroxyapatites by its distinctive x-ray diffraction pattern as shown in FIG. 1. Unlike the ideal stoichiometric crystalline hydroxyapatite, Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2, with atomic Ca/P ratio of 1.67, the composition of bone mineral is significantly different and may be represented by the following formulae, EQU Ca.sub.8.3 (PO.sub.4).sub.4.3 (HPO.sub.4, CO.sub.3).sub.1.7 (OH,CO.sub.3).sub.0.3.
Bone mineral non-stoichiometry is primarily due to the presence of divalent ions, such as CO.sub.3.sup.2- and HPO.sub.4.sup.2-, which are substituted for the trivalent PO.sub.4.sup.3- ions. Substitution by HPO.sub.4.sup.2- and CO.sub.3.sup.2- ions produces a change of the Ca/P ratio, resulting in Ca/P ratio which may vary between 1.50 to 1.70, depending on the age and bony site. Generally, the Ca/P ratio increases during aging of bone, suggesting that the amount of carbonate species typically increases for older bones. It is the Ca/P ratio in conjunction with nanocrystalline size and the poorly-crystalline nature that yields specific solubility property of the bone minerals. And because bone tissues undergo constant tissue repair regulated by the mineral-resorbing cells (osteoclasts) and mineral-producing cells (osteoblasts), solubility behavior of minerals is important in maintaining a delicate metabolic balance between these cell activities.
Synthetic bone graft material made to closely resemble natural bone minerals can be a useful replacement for natural bone. Acceptable synthetic bone can avoid the problem of availability and harvesting of autologous bone (patient's own bone) and the risks and complications associated with allograft bone (bone from a cadaver), such as risks of viral transmission. An ideal synthetic bone graft should possess a minimum of the following four properties: (1) it should be chemically biocompatible; (2) it should provide some degree of structural integrity in order to keep the graft in place and intact until the patient's own bone heals around it; (3) it should be resorbable so that the patient's own bone ultimately replaces the graft; and, (4) because it may be necessary to incorporate cells and/or biomolecules into the synthetic bone material, it is desirable that the process used to form the material employ low temperatures and chemically mild conditions. Similar criteria are also important for other hard tissue grafts (e.g. cartilage).
These criteria may be met by a material in which parameters, such as Ca/P ratios, crystal size, crystallinity, porosity, density, thermal stability and material purity are controlled. While there have been considerable attempts to synthesize a ceramic material which closely resembles natural bone for use as implants, synthetic hydroxyapatite has traditionally been the preferred choice. Previous bone ceramics often involved stoichiometric apatites with significant crystalline form often with larger crystal sizes. The prior art (LeGeros R. Z., in Calcium Phosphates in Oral Biology and Medicine, Karger Pub. Co., New York, 1991) teaches that highly crystalline form of hydroxyapatite is produced by solution precipitation followed by sintering at high temperatures (800-1200.degree. C.). High temperature treatment yields highly stoichiometric hydroxyapatite with crystal sizes on the order of several microns with Ca/P of 1.67. Such highly crystalline hydroxyapatite has an extremely low solubility rendering it essentially insoluble in the host tissue. Therefore, it is not replaced by living bone tissue and it remains intact in the patient for an extended period.
A bone growth implant should possess sufficient mechanical strength to support the bone. But it is preferably bioerodible so that the bone will eventually support its own weight. This is a problem with metal pins which are commonly used to bind a fracture together. Although the pins possess sufficient mechanical strength to support the bone as it heals, this strength never diminishes and the bone never strengthens to a point that is it able to bear its body weight. The development of a material which would provide the requisite mechanical strength and a satisfactory bioerosion rate is desired.
A number of calcium phosphate bone fillers and cements have been referred to as "bioresorbable." Generally, these are compounds comprising or derived from tricalcium phosphate, tetracalcium phosphate or hydroxyapatite. These materials all have significantly greater crystalline character than the poorly crystalline apatitic calcium phosphate found in bone. At best these materials may be considered only "weakly" resorbable. Of these, the tricalcium phosphate compounds have been demonstrated to be the most resorbable and after many years of study they are still not widely used in clinical settings. The tricalcium phosphates are known to have lengthy and somewhat unpredictable resorption profiles, generally requiring in excess of one year for resorption. Furthermore, unless steps are taken to produce extremely porous or channeled samples, the tricalcium phosphates are not replaced by bone. Recently it has been concluded that the "biodegradation of TCP, which is higher than that of Hap [hydroxyapatite] is not sufficient" (Berger et al., Biomaterials, 16:1241 (1995)). Tetracalcium phosphate and hydroxyapatite derived compounds are also only weakly resorbable. Tetracalcium phosphate fillers generally exhibit partial resorption over long periods of time such as 80% resorption after 30 months (Horioglu et al., Soc. for Biomaterials, March 18-22, pg 198 (1995)).
There remains a need for a synthetic bone material that more closely mimics the properties of naturally occurring minerals in bone. In particular, there remains a need to provide synthetic bioceramics which are completely bioresorbable and biocompatible. The use of such a resorbable calcium phosphate in biomedical devices provides many advantages over alternative conventional materials. For instance, it eliminates the need for post-therapy surgery to remove the device and degrades in the human body to biocompatible, bioresorbable products.